Composites are defined broadly as the combination of two or more dissimilar materials to produce a new material that has synergistic properties that were not present in the individual constituents alone. In practical terms, the word composite is generally associated with reinforced plastic material such as fiberglass structures. In the case of fiberglass, beneficial synergistic properties including corrosion resistance, low weight, high strength, and low cost are attainable in a highly variable array of product geometries.
Fabrication of a composite article such as a fiberglass boat hull requires the combination of a solidifiable resin system with a “preform” that could include glass fibers, veils, flow media and cores. There are many processes available for the purpose of impregnating a preform with liquid resin in order to make a composite. These processes may be broadly characterized into two categories, wet lay-up “open molding” and resin infusion “closed molding.”
Open molding processes tend to produce a final component having a low fiber volume fraction (i.e., lower relative amount of fiber compared to the amount of resin). They are also labor intensive to manufacture because each layer of preform material must be individually coated with resin and carefully positioned by hand. Further, the inherent nature of open molding processes can allow air bubble entrapment to occur inside the composite, and the completed part can have a non-uniform thickness and fiber volume fraction.
In addition, open molding often leads to direct worker exposure to Volatile Organic Compounds (VOC) and Hazardous Airborne Pollutants (HAP). Both VOC and HAP are recognized by the EPA as potential health hazards for which alternative control technologies should be sought. As a result, although exceptions can be found, these deficiencies generally result in articles formed by open molding techniques being disfavored where other methods are available.
By comparison, closed molding—and more particularly resin transfer molding (RTM)—overcomes many of the limitations of wet lay-up processes. RTM involves a preform being constrained under pressure within a mold cavity whereupon resin is forced into the open spaces remaining. Resin infusion methods limit exposure to VOC and HAP and allow for better control over part dimensions and fiber volume fraction. RTM molds are typically made from matched steel mold platens which are supported in a hydraulic press due to the high injection pressures required to force resin through a highly compacted preform. The escalating cost of fabricating rigid molds for parts in excess of about 100 square feet tends to limit the size of parts considered for RTM.
Vacuum Assisted Resin Transfer Molding (VARTM) is a variation of RTM that achieves preform compaction by removing air located between a single sided rigid tool and a flexible vacuum bag that encapsulates a preform placed on the tool. Tooling costs are significantly reduced because there is only one tool surface and atmospheric pressure replaces the hydraulic press. VARTM provides a closed mold solution for complex and/or large parts that were previously not considered infusable. A desirable element of a VARTM mold is a vacuum bag that has sufficient elasticity to accommodate the strains associated with preform compaction as air is removed. It is further desirable for the vacuum bag to be sufficiently impermeable so that air does not leak through the bag and adversely affect the flow of resin or leave air pockets within the composite product. A vacuum bag should also provide a sufficiently snug fit around a preform to prevent the formation of creases and/or bridges which can become resin runners leading to inconsistent flow fronts.
The most common vacuum bag currently used for VARTM is a single-use Nylon film, and variations are available with more or less stretch, heat resistance, tear strength and thickness. Films are sold in flat sheet stock requiring fabricators to cut, paste and seam sections together as needed to build a suitable vacuum bag. While suppliers are now offering the convenience of thermally seamed near net shape film bags, Nylon films are not reusable and thus end up in the dump after each mold run. Furthermore, disposable bags of this kind rarely provide sufficient elasticity to eliminate bag bridging and or bulging which can lead to inconsistent infusions and dry spots in the molded composite article.
The composites industry is beginning to recognize that reusable vacuum bags are a desirable component of economically viable production closed molding programs, with bag longevity being a key factor. Reusable bags must withstand significantly more wear and abuse than disposable bags. A variety of Synthetic rubbers have been used to make reusable vacuum bags, including calendared rubber sheets of EPDM, Silicone, butyl, fluoroelastomers, nitriles and polyisoprenes and room temperature vulcanizing (RTV) silicones, all of which originate from a petroleum feed stock.
For reasons of transparency and the ability to make near net shape constructions, RTV silicone systems have become the material of choice for making reusable vacuum bags. Vacuum bags made from calendared silicone sheets require seam treatments of either RTV silicone or a beta staged silicone material that must be subsequently cured with heat and moisture. Reusable bags are also made from semi-cured silicone sheet stock that is cut into desired shapes, draped in place on the mold surface upon which the seams are troweled over to create low profile joints. Another method involves spreading an uncured thixotropic RTV silicone liquid uniformly over a mold surface prior to curing it. In all of these instances, the procedure for building a reusable silicone vacuum bag is tedious and requires skilled labor.
Silicones have poor puncture and tear resistance, however, and therefore must be reinforced or thickened for durability, which makes them susceptible to the bridging effect in addition to being unnecessarily heavy and cumbersome to manipulate. For very large parts such as boat hulls, bridge decks, and wind blades, the weight of a given bag can become a significant issue. Large bags often need to be lifted mechanically and therefore require lift points. Bag strength becomes a critical factor because thicker bags weigh more and droopy bags can get caught on foreign objects and become damaged.
Attempts have been made to spray silicone rubbers with mixed results. Typical RTV silicones have high viscosity and are thixotropic which makes them difficult to spray because the material does not flow easily. It is thus difficult to achieve uniform bag thicknesses over large areas because the product must be toweled out after being applied to the surface. It is possible to reduce the viscosity of RTV silicones with the addition of solvents, but this remedy has the potential to become a source of VOC and HAP. Spray equipment that atomizes the silicone also runs the risk of contaminating the surfaces of neighboring articles exposed to the overspray and can become a major problem for adhesive bonding and/or painting operations carried out in the vicinity.
In light of the factors that should be considered when fabricating a composite article, there still exists a need for a durable, reusable vacuum bag for use in closed molding and vacuum bagging applications that limits the production of VOC and HAP and minimizes the overall environmental impact.